1. Field of the Invention
The present invention relates to a pressure accumulation type of fuel injection device which has a pressure accumulation chamber defined in a fuel injector and adapted to store each incoming charge of highly pressurized fuel delivered from a fuel injection pump and is adapted to open a nozzle valve by the fuel pressure in the pressure accumulation chamber at the completion of the delivery stroke of the fuel injection pump so as to inject the fuel under high pressure through spray holes.
2. Background of the Prior Art
It is known in the art, for example per U.S. Pat. No. 4,561,590 and U.S. Pat. No. 4,436,247, that to have a pressure accumulation type of fuel injection device wherein each incoming charge of highly pressurized fuel delivered from a fuel injection pump is adapted to be stored temporarily in a pressure accumulation chamber defined in a fuel injector to thereafter be injected into a combustion chamber of an internal combustion engine at the completion of the delivery stroke of a fuel injection pump.
The device disclosed in U.S. Pat. No. 4,561,590 includes a nozzle valve arranged in a pressure accumulation chamber so as to close spray holes and a check valve arranged therein so as to block an inflow passageway into the pressure accumulation chamber. Between the upper end of the nozzle valve and the check valve there is provided a compression spring so that the valve stem of the check valve can slide reciprocatingly within a blind hole formed at the upper side of the nozzle valve. The nozzle valve can thus be reduced in weight to enhance the effectiveness. However, the device is constructed so that the blocking force of the check valve and the closing force of the nozzle valve are obtained by a single compression spring. The spring force of this spring therefore has to be very strong because it sets the fuel injection pressure. Therefore, some problems arise, e.g., it is impossible to store the highly pressurized fuel in the pressure accumulation chamber because the closing force of the check valve also becomes correspondingly very strong. On the other hand, since the upper end of the nozzle valve is arranged to be in communication with a fuel passageway (an inlet chamber) disposed at the upstream of the check valve via a throughhole formed within the stem of the check valve, the pressure receiving area for closing the nozzle valve becomes larger than that for opening it. Therefore, both the valve opening timing and the valve closing timing of the nozzle valve become unharmonious because small pressure variations in the inlet chamber have significant influence on the opening and the closing force for the nozzle valve.
On the other hand, the device disclosed in U.S. Pat. No. 4,436,247 has a cylindrical check valve fitted hermetically and slidably around a valve stem of a nozzle valve which is arranged within a pressure accumulation chamber, and a compression spring provided between the nozzle valve and the check valve. The upper end of the nozzle valve is adapted to extend to a fuel inflow passageway (an inlet chamber) disposed at the upstream of the check valve. Also, in this case, since the closing forces for the nozzle valve as well as the check valve are adapted to be obtained by a single compression spring, it is impossible to increase the fuel pressure which is stored in the pressure accumulation chamber. Further, since the upper end of the nozzle valve stem is extended into the inlet chamber, and the pressure receiving area for closing the nozzle valve becomes larger than that for opening it, the pressure variations in the inlet chamber influence the operating timing of the nozzle valve, that is, the opening timing and the closing timing of the nozzle valve become unpredictably unharmonious.